In general, a hybrid excavator includes a hydraulic pump motor for moving a boom upward and downward, an electric motor, which implements power generation and power transmission and is connected to one side of the hydraulic pump motor, and an electric energy storage device such as an ultra-capacitor, which is charged with generated electric power, at the other side of the electric motor.
In addition, a hydraulic fluid discharged from the hydraulic pump motor is provided to the boom via a boom control valve, and by control of the boom control valve, the boom is moved upward, stopped, or moved downward.
The aforementioned configuration of the hybrid excavator will be described in more detail with reference to the attached FIG. 1.
A boom actuator 100 is connected to a boom control valve 125, and the boom control valve 125 is connected to a hydraulic pump motor 120.
The boom control valve 125 has three positions, and the boom control valve 125 allows the boom actuator 100 to perform an upward operation at a first position 126, allows the boom actuator 100 to perform a downward operation at a second position 127, allows the boom actuator 100 to stop the upward and downward operations at a third position 128 that is a neutral position.
The hydraulic pump motor 120 may serve as both a hydraulic pump and a hydraulic motor.
A discharge line 121 and an inlet line 122 are connected to the hydraulic pump motor 120. In addition, the other side of the discharge line 121 and the other side of the inlet line 122 are connected to the boom control valve 125.
In addition, a first control valve 151 is connected to one side of the inlet line 122 on a route that is connected to a drain tank. The first control valve 151 is controlled to be closed by the downward operation of the boom actuator 100 when regenerative energy is collected, and controlled to be opened to discharge the hydraulic fluid when regenerative energy is not collected, or when a flow rate of the hydraulic pump motor 120 exceeds a permissible flow rate.
In addition, a second control valve 152 is connected to one side of the discharge line 121 on a route that is connected to the drain tank. The second control valve 152 is controlled to be closed when the boom is moved upward, and controlled to be opened to discharge the hydraulic fluid when the boom actuator 100 performs the downward operation.
In addition, a motor bypass valve 200, which is connected to the discharge line 121 and the inlet line 122, is provided, and the motor bypass valve 200 connects or disconnects the discharge line 121 and the inlet line 122.
On the other hand, one side of a boom auxiliary line 145 may be connected to the discharge line 121, and a boom auxiliary valve 144 may be provided at the other side of the boom auxiliary line 145. The boom auxiliary valve 144 is controlled to add and supply the hydraulic fluid from a main hydraulic pump to the discharge line 121.
The aforementioned boom driving system for a hybrid excavator in the related art has the following problems.
FIG. 1 illustrates a case when assuming that a permissible flow rate of the hydraulic pump motor is larger than a regenerative flow rate in the boom driving system.
A high-pressure fluid (hydraulic fluid) at a head side of a boom cylinder of the boom actuator 100 is transmitted to an intake side of the hydraulic pump motor 120. The hydraulic pump motor 120 implements a hydraulic motor function by pressurized oil (hydraulic fluid), and rotates the electric motor. As a result, the electric motor regenerates electric energy from potential energy of the boom, and the electric energy storage device is charged with electric energy.
A low-pressure hydraulic fluid passing through the hydraulic pump motor 120 is supplied to a rod side of the boom cylinder of the boom actuator 100, and a surplus amount of hydraulic fluid due to a difference in cylinder area is discharged to the drain tank via the second control valve 152.
When the boom is moved downward, a retraction speed of the boom actuator 100 is controlled by a rotational speed of the boom electric motor. That is, as illustrated in FIG. 2A, the rotational speed of the electric motor is increased proportionally to boom downward movement joystick pressure.
In a case in which the amount and pressure of hydraulic fluid, which is supplied from a boom head side of the boom actuator 100, are sufficient, the boom electric motor is operated by the hydraulic pump motor 120 that is operated as a hydraulic motor, and in this case, the electric motor implements a generator function, such that torque of the electric motor has a minus (−) value, as illustrated by a solid line indicated in FIG. 2B.
However, when the boom of the excavator is moved downward, for example, when the excavator performs excavation work on the slope, the amount and pressure of hydraulic fluid, which is supplied from the boom head side of the boom actuator 100, are insufficient. Accordingly, power, which is supplied from the boom cylinder of the boom actuator 100 to the hydraulic pump motor 120, may be insufficient.
The electric motor is operated as an electric motor using electric power from the electric energy storage device (capacitor), as illustrated by a dotted line indicated in FIG. 2B, so as to be rotated at a desired rotational speed, as illustrated in FIG. 2A, and in this case, torque of the electric motor has a plus (+) value.
High pressure needs to be formed at the cylinder rod side of the boom actuator 100 in order to implement a predetermined speed or more at which the boom actuator is retracted in a case in which the boom of the excavator is moved downward. However, the electric motor may be rotated at a target speed in the boom driving system for a hybrid excavator in the related art, but pressure in the discharge line 121 is maintained to be low because the discharge line 121 is connected to the drain tank via the second control valve 152.
Accordingly, there is a problem in that a speed at which the rod of the boom actuator 100 is retracted and force by which the rod of the boom actuator 100 is retracted cannot be controlled to be increased.